Default to spec.subdomain for automated hostname creation in Route API

1. Proposed title of this feature request
Default to spec.subdomain for automated hostname creation in Route API

2. What is the nature and description of the request?
Post NE-882 respectively Route Subdomain it would be useful to either default to spec.subdomain for creating automated hostname in the Route API or otherwise have a way to enable such functionality on per IngressController level or generally in OpenShift Container Platform 4. Specifically for environemnts where router sharding is heavily used, such functionality would become handy to ease management of hostname created by the Route API and also ease the work for 3rd party solutions and/or layered products provided by Red Hat.

3. Why does the customer need this? (List the business requirements here)
Right now, automated hostname created via Route API always use the default domain set for the OpenShift Container Platform 4 - Cluster, despite a specific domain may be configured on the respective IngressController . If spec.subdomain could be used as default, it would make hostname creation via Route API easier and allow a more flexible approach, where the correct domain will be used upon hostname creation via Route API. Hence also obsolte to potentially specify the domain in custom resources as it can be picked up via IngressController automatically.

4. List any affected packages or components.

• Route API
• IngressController
• IngressOperator